Spacerless artificial disc replacements

ABSTRACT

Spacerless artificial disc replacements (ADR) are disclosed. One preferred embodiment includes two saddle-shaped components to facilitate more normal spinal flexion, extension, and lateral bending while limit axial rotation, thereby protecting the facet joints and the annulus fibrosus (AF). Either or both of the superior and inferior components are made of a hard material such as chrome cobalt, titanium, or a ceramic including alumina, zirconia, or calcium phosphate. The articulating surfaces of the ADR are also preferably highly polished to reduce friction between the components. Metals, alloys or other materials with shape-memory characteristics may also prove beneficial.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/372,520, filed Apr. 12, 2002 and this application is a continuation of U.S. application Ser. No. 10/413,028, filed on Apr. 14, 2003, the disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to artificial disc replacements (ADRs) and, more particularly, to devices that operate without softer spacer materials such as polyethylene.

BACKGROUND OF THE INVENTION

Polyethylene spacers are common in some artificial joint situations, including total knee replacements (TKRs). Polyethylene spacers are also used between metal plates in many artificial disc replacement (ADR) designs.

Complications arising from poly debris are well documented, however, including fracture of the spacer once it becomes too thin, absorptions and migration of poly particles throughout the body, and loosening of the bone metal junction as a reaction of the poly debris.

Metal-on-metal and ceramic-on-metal surfaces have much lower wear characteristics. In fact, metal-on-metal surfaces demonstrate 400 times less wear than polyethylene on metal surfaces.

While there have been attempts to limit the use of the poly in ADR designs, all existing approaches constitute call-and-socket configurations which do not inherently limit axial rotation. Instead, axial rotation is limited through the use of multiple ball-and-socket joints or an elongated ball-and-socket joint, which complicates the design.

SUMMARY OF THE INVENTION

The present invention replaces polyethylene artificial disc replacement (ADR) spacers with harder, more wear resistant materials. In the preferred embodiments, an ADR according to the invention includes opposing saddle-shaped components to facilitate more normal spinal flexion, extension, and lateral bending. Preferably, the ADR allows at least 10 degrees of movement on the flexion to extension direction and at least 5 degrees of movement in the lateral bending direction. The saddle-shaped articulating surfaces also limit axial rotation, thereby protecting the facet joints and the annulus fibrosis (AF).

According to the invention, either or both the superior and inferior components are made of a hard material such as chrome cobalt, titanium, or a ceramic including alumina, zirconia, or calcium phosphate. The articulating surfaces of the ADR are also preferably highly polished to reduce friction between the components. Metals, alloys or other materials with shape-memory characteristics may also prove beneficial.

The vertebral surfaces of the components may be treated to promote bone ingrowth. For example, the vertebral surfaces of the components may have plasma spray or beads. Alternatively, one or both components may be cemented to the vertebrae. The vertebra-facing surfaces may also include projections such as keels that fit into the vertebrae. In embodiments adapted for cementation, one of the components could be made of polyethylene or other softer material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an anterior view of an ADR according to the invention;

FIG. 1B is a lateral view of the ADR of FIG. 1;

FIG. 1C is an oblique view of the ADR of FIG. 1;

FIG. 2A is a view of the anterior aspect of an alternative embodiment of the ADR;

FIG. 2B is a view of the lateral aspect of an alternative embodiment of the ADR shown in FIG. 2B;

FIG. 3A is a view of the anterior aspect of an alternative, less constrained, embodiment of the saddle-shaped ADR shown in a fully flexed position;

FIG. 3B is a view of the lateral aspect of the embodiment of the ADR shown in FIG. 3A;

FIG. 4A is a view of the lateral aspect of another embodiment of a saddle-shaped ADR;

FIG. 4B is a view of the anterior aspect of the embodiment of the ADR shown in FIG. 4A in a fully flexed position;

FIG. 4C is a view of the anterior aspect of an alternative embodiment;

FIG. 4D is a view of the anterior aspect of the ADR shown in FIG. 4A; and

FIG. 4E is a view of the anterior aspect of the ADR shown in FIG. 4A.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is an anterior view of an ADR according to the invention. FIG. 1B is a lateral view of the ADR of FIG. 1. FIG. 1C is an oblique view of the ADR of FIG. 1.

FIG. 2A is a view of the anterior aspect of an alternative embodiment of the ADR, wherein the articulating surfaces of both components have a flat area centrally from the front to the back of the ADR. FIG. 2B is a view of the lateral aspect of an alternative embodiment of the ADR drawn in FIG. 2B. The flat area of the articulating surfaces courses centrally from side of the DR to the other side. The flat area allows one component to translate slightly on the other component. Alternatively, a curved area with a large radius could replace the flat area.

FIG. 3A is a view of the anterior aspect of an alternative, less constrained, embodiment of the saddle-shaped ADR drawn in a fully flexed position. The less constrained embodiment facilitates spinal flexion, extension, and lateral bending. FIG. 3B is a view of the lateral aspect of the embodiment of the ADR drawn in FIG. 3A.

FIG. 4A is a view of the lateral aspect of another embodiment of a saddle-shaped ADR. The center of rotation for flexion and extension is not necessarily located in the center of the ADR. For example, the center of rotation is preferably located in the posterior half of the ADR. FIG. 4B is a view of the anterior aspect of the embodiment of the ADR drawn in FIG. 4A, drawn in a fully flexed position.

FIG. 4C is a view of the anterior aspect of an alternative embodiment, showing how the radius of curvature of the articulation for lateral bending may be different than the radius of curvature for articulation for flexion and extension. For example, the radius of curvature for the articulation for flexion and extension, as seen in FIG. 4A, may be smaller than the radius of curvature for the articulation for lateral bending, as seen in FIG. 4C. Articulating surfaces with smaller radii, facilitate movement. Thus, the embodiment of the ADR drawn in FIG. 4A flexes and extends more easily than the embodiment of the ADR drawn in FIG. 3A. The ADR is drawn in a fully flexed position.

FIG. 4D is a view of the anterior aspect of the ADR drawn in FIG. 4A. The ADR is drawn in a neutral position. The area of the drawing with diagonal lines represents the articulating surface of the lower ADR component. FIG. 4E is a view of the anterior aspect of the ADR drawn in FIG. 4A, also drawn in a fully extend position. 

1. An artificial disc replacement comprising: a first component having a first articulating surface, said first articulating surface being a single saddle-shaped surface; and a second component having a second articulating surface in contact with said first articulating surface, said second articulating surface being a single saddle-shaped surface, wherein said first and second components are operable to articulate relative to one another about at least one of (i) a first center of rotation located above said first and second articulating surfaces for one of flexion/extension or lateral bending, and (ii) a second center of rotation located below said first and second articulating surfaces for the other of flexion/extension or lateral bending.
 2. The artificial disc replacement of claim 1, wherein said first and second articulating surfaces engage one another.
 3. The artificial disc replacement of claim 1, wherein said first and second articulating surfaces include substantially congruent surfaces.
 4. The artificial disc replacement of claim 1, wherein said first articulating surface includes a single substantially continuous concave surface and a single substantially continuous convex surface, and said second articulating surface includes a single substantially continuous concave surface and a single substantially continuous convex surface.
 5. The artificial disc replacement of claim 4, wherein the concave surface of said first articulating surface engages the convex surface of said second articulating surface, and the convex surface of said first articulating surface engages the concave surface of said second articulating surface.
 6. The artificial disc replacement of claim 4, wherein the single concave and convex surfaces of said first articulating surface extend between outer edges of said first articulating surface, and the single concave and convex surfaces of said second articulating surface extend between outer edges of said second articulating surface.
 7. The artificial disc replacement of claim 1, wherein said first articulating surface is defined by a concave arc having a substantially constant radius of curvature A about a first axis, and a convex arc having a substantially constant radius of curvature B about a second axis perpendicular to the first axis, and said second articulating surface is defined by a convex arc having a substantially constant radius of curvature C about a third axis, and a concave arc having a substantially constant radius of curvature D about a fourth axis perpendicular to the third axis.
 8. The artificial disc replacement of claim 7, wherein said radius of curvature A is substantially equal to said radius of curvature C and said radius of curvature B is substantially equal to said radius of curvature D.
 9. The artificial disc replacement of claim 7, wherein at least one of: (i) said first and third axes are coaxial; and (ii) said second and fourth axes are coaxial.
 10. The artificial disc replacement of claim 7, wherein at least one of: (i) said first and third axes are coplanar; and (ii) said second and fourth axes are coplanar.
 11. The artificial disc replacement of claim 1, wherein said first and second member further include projections for engaging a vertebral body.
 12. The artificial disc replacement of claim 11, wherein said projections are keels.
 13. The artificial disc replacement of claim 11, wherein said projections are selected from the group consisting of flanges, keels and spikes.
 14. The artificial disc replacement of claim 1, wherein said first and second components are constructed of hard materials.
 15. The artificial disc replacement of claim 14, wherein said hard materials are selected from the group consisting of chrome cobalt, titanium, alumina, zirconia, and calcium phosphate.
 16. The artificial disc replacement of claim 1, wherein said first and second components each further include vertebral surfaces for contacting vertebral bodies.
 17. The artificial disc replacement of claim 16, wherein said vertebral surfaces have plasma spray or beads thereon.
 18. The artificial disc replacement of claim 1, wherein said first component is cemented to a first vertebral body and said second component is cemented to a second vertebral body. 